It is well known that the existence of oxygen in the sintering of aluminium nitride (AlN) causes the degradation of thermal conductivity. To clarify the role of the oxygen, the effect of Al2O3 addition on the phase reaction of the AlN-Y2O3 system at high temperatures of 1900 and 1950 degrees C was investigated. In the AlN-Al2O3 system, gamma-AlON and 27R polytypoid were formed at 1900 and 1950 degrees C, respectively, and the ratio of each crystal increased with increasing Al2O3 content. The resultant phase was found to be in agreement with the phase diagram given by McCauley and Corbin. In the AlN-Y2O3 system, YAM phase or YAM plus unreacted Y2O3 were identified. Phase reaction in the AlN-Al-23-2Y(2)O(3) system was quite different from that of the AlN-Al2O3 system : namely, gamma-AlON and 27R coexisted even at 1950 degrees C, which is inconsistent with McCauley and Corbin’s diagram. It was confirmed by the dilatometric method and phase reaction data using some couples, that Al2O3 strongly affected the phase reaction in the AlN-Y2O3 system, indicating that the shrinkage of the Al2O3-doped specimen was initiated at somewhat lower temperatures. The grain morphology depended on the phase changes shown above, in which AlN and gamma-AlON were granular, and 27R polytypoid was plate-like.